1. Field of the Invention
The present invention relates to an internal combustion engine variable compression ratio system, and in particular to an improvement thereof in which a piston includes an inner piston and a outer piston. The inner piston is connected to a connecting rod via a piston pin, and the outer piston being fitted slidably around the outer periphery of the inner piston and having a head portion facing a combustion chamber. An operating device disposed between the inner piston and the outer piston moves and holds the outer piston relative to the inner piston alternately at a low compression ratio position close to the piston pin and at a high compression ratio position close to the combustion chamber, thereby making the engine compression ratio variable.
2. Background Art
As a conventional internal combustion engine variable compression ratio system, there is a known system (1) in which an outer piston is screwed around the outer periphery of a inner piston, and the outer piston is rotated forward and backward so that it approaches and recedes from the inner piston to move to a low compression ratio position and a high compression ratio position (for example, Japanese Patent Application Laid-open No. 11-117779).
Another known system (2) includes an outer piston is fitted in an axially slidable manner around the outer periphery of a inner piston, an upper hydraulic chamber and a lower hydraulic chamber are formed between the inner piston and the outer piston, and supply of hydraulic pressure alternately to these hydraulic chambers moves the outer piston to a low compression ratio position and a high compression ratio position (for example, Japanese Patent Publication No. 7-113330).
However, in the above-mentioned system (1), since it is necessary to rotate the outer piston in order to move it to the low compression ratio position and the high compression ratio position, the shape of the top face of the outer piston cannot be set freely so as to match the shape of the roof of a combustion chamber and the positional arrangement of intake and exhaust valves, and it is difficult to sufficiently increase the compression ratio of the engine at the high compression ratio position. Furthermore, in the above-mentioned system (2), particularly when the outer piston is at the high compression ratio position, since a large thrust load acting on the outer piston during an expansion stroke of the engine is borne by the hydraulic pressure of the upper hydraulic chamber, it is necessary for the upper hydraulic chamber to have a seal that can withstand high pressure, and moreover if bubbles are generated in the upper hydraulic chamber, the high compression ratio position of the outer piston becomes unstable, so that it is necessary to provide means for removing such bubbles, thus inevitably increasing the overall cost.